1. Field of the Invention
The present invention relates generally to a PM (permanent magnet) type step motor used to move a camera or an optical pickup lens of media related equipment and, more particularly, to a step motor wherein coupling holes are formed in the central portions of both sides of a casing unit thereof, a bushing is press-fitted or inserted into an associated coupling hole, and a ring member is press-fitted over the outer circumference of the bushing protruding outward from the coupling hole such that the bushing is secured to the casing unit, thus preventing the bushing from becoming separated from the casing unit, and such a step motor can be freely applied to peripheral device modules of various outer diameters using the ring member.
2. Description of the Related Art
Generally, a PM (permanent magnet) type step motor includes a rotor to which magnets are attached, and a stator which is spaced apart from the outer circumferences of the magnets by a predetermined interval, so that the step motor rotates in response to an external electric signal applied to the stator.
The step motor includes the rotor and the stator, with a casing provided around the outer circumference of the stator to protect and cover the stator.
The rotor includes a plurality of magnets which are provided on the outer circumference of a rotating shaft and are magnetized such that their N and S poles alternate, the rotor being inserted into the inside of the stator. The stator includes a bobbin which receives the coil and yokes which have a plurality of yoke teeth inserted into the inside of the bobbin.
The yokes are constructed such that the plurality of yoke teeth face and engage with each other. The magnetic path of the magnets is formed to pass through the yoke teeth which engage with each other, so that the rotor of the step motor is rotated by external power applied to the stator.
The step motor includes one pair of bobbins which are coupled to face each other. A terminal block protrudes from the upper portion of each bobbin, and protects a connection terminal pin which applies external power to the coil received in the bobbin.
In the conventional PM type step motor, two pairs of yoke teeth which face each other protrude axially. A bushing for supporting the shaft is press-fitted into each of the opposite ends of the casing which surrounds and protects the yoke teeth, or the bushing is secured using a cover having a hollow portion and then is attached to the casing.
Here, the cover and the casing do not reliably support the bushing, so that the bushing may become separated. In order to solve the problem, the bushing is secured by caulking or a similar process. However, the caulking process may crush the bushing, so that the bushing may be distorted. Further, the bushing may be deformed by the caulking and press-fitting. It is difficult to concentrically arrange the bushings provided on the opposite ends of the stator and the stator.
Further, the outer diameter of the bushing protruding from the casing may be used as a matching reference when the step motor is used in a camera or a media related equipment module. However, in order to make it the required size, the construction of the casing must be changed. In this case, constraints are imposed on the method of locking the bushing.